100 DAYS OF SCIENCE: Tucson's Planetary Science Institute Crafted Moon Theory

By Beal, Tom | AZ Daily Star, August 24, 2012 | Go to article overview

100 DAYS OF SCIENCE: Tucson's Planetary Science Institute Crafted Moon Theory


Beal, Tom, AZ Daily Star


The Arizona Daily Star's Centennial salute to science in Arizona runs all summer. Each day, for 100 days, we'll record a milestone in the state's scientific history.

Where did our moon come from?

Here's the short answer about the "giant impact hypothesis" from the Tucson-based Planetary Science Institute:

"At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit Earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon."

If you already knew that, your knowledge comes courtesy of two Tucson planetary scientists, who proposed the theory in 1974.

Donald R. Davis and William K. Hartmann are founding members of the Planetary Science Institute, which celebrates its 40th anniversary this year.

Hartmann, who developed an expertise in cratering of the moon as he helped Gerard Kuiper map it as a graduate student at the University of Arizona, is also an astronomical illustrator who provided the image that accompanies this article.

Davis is a "dynamicist" who helped develop the NASA computer programs that provided safe-return paths for the Apollo missions under a variety of circumstances - information that became critical to the crippled Apollo 13 flight.

He developed a set of "accretion models" that showed how planets might evolve from discs of debris. "They were models we could run on computers, which was neat because those were things people hadn't been able to do before that time. …

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